Is there more than one universe? Physicist Brian Greene shows how the unanswered questions of physics (starting with a big one: What caused the Big Bang?) have led to the theory that our own universe is just one of many in the "multiverse."

About Brian Greene

Brian Greene is perhaps the best-known proponent of superstring theory, the idea that minuscule strands of energy vibrating in a higher dimensional space-time create every particle and force in the universe. Greene, a professor of physics and mathematics at Columbia University, has focused on unified theories for more than 25 years, and has written several best-selling and non-technical books on the subject including The Elegant Universe, a Pulitzer finalist, and The Fabric of the Cosmos—each of which has been adapted into a NOVA mini-series. His latest book, The Hidden Reality, explores the possibility that our universe is not the only universe.

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GUY RAZ, HOST:

Okay so, about 400 billion stars, and at least that many planets, in our own Milky Way galaxy, right? But what if we could go beyond that radius, or even further? What if the universe, the totality of existence, isn't actually everything? Well, at the beginning of the show, we heard the story about the Nobel Prize winning discovery that changed our understanding of the universe. The discovery in the late 1990s that the expansion of the universe wasn't slowing down, as everyone believed, but that it was getting faster every second.

BRIAN GREENE: And to learn that it was speeding up, literally it's as if someone said to you, throw an apple up in the air and it's going to go up faster and faster and faster and faster and faster...

RAZ: But, why? Brian Greene, a physicist at Columbia University, asked that question in his TED Talk. If you threw an apple up into the sky, and it kept going up, but getting faster and faster...

(SOUNDBITE OF BRIAN GREENE TED TALK)

GREENE: ...you'd want to know why. What's pushing on it? Similarly, the astronomers' results are surely well deserving of the Nobel Prize, but they raised an analogous question: What force is driving all galaxies to rush away from every other at an ever-quickening speed? Well, the most promising answer comes from an old idea of Einstein's. You see, we are all used to gravity being a force that does one thing, pulls objects together. But in Einstein's theory of gravity, his general theory of relativity, gravity can also push things apart. How? Well, according to Einstein's math, if space is uniformly filled with an invisible energy - sort of like a uniform, invisible mist - then the gravity generated by that mist would be repulsive.

RAZ: And there's a number, right? There's a specific number, amount of dark energy - we've measured this - that we can associate with our universe.

GREENE: That's right, and that's the, the big, huge mystery to us. We've measured the enough amount of dark energy, assuming that's the right explanation, and it's a decimal point followed by roughly 122 zeros and then a one, which is such a strange number. It's the kind of number that we don't typically encounter when we do physics or mathematics.

(SOUNDBITE OF BRIAN GREENE TED TALK)

GREENE: This number is small. Expressed in the relevant units, it is spectacularly small and the mystery is to explain this peculiar number. We want this number to emerge from the laws of physics, but so far, no one has found a way to do that. Now you might wonder, should you care? Maybe explaining this number is just a technical issue, a technical detail of interest to experts but of no relevance to anybody else. Well it surely is a technical detail but some details really matter. Some details provide windows into uncharted realms of reality and this peculiar number may be doing just that, as the only approach that's so far made headway to explain it invokes the possibility of other universes.

RAZ: Other universes, and by focusing on that number, scientists might not be asking the right question.

GREENE: Kepler, back many centuries ago, asked a seemingly natural question, which is, why is the Earth 93 million miles from the sun?

RAZ: Back in the 17th century, Johannes Kepler was obsessed with this question.

GREENE: 93 million miles. 93 million miles.

Why is the earth 93 million miles from the sun? He wanted to find some explanation where he could do some mathematics and, at the end of it, out would pop 93 million miles.

RAZ: So as if there was some deep, sacred law in the universe that could explain that number, like the dark energy number?

GREENE: That's right and he looked for explanations, but never found any. And now we know that he was, in fact, asking the wrong question because there is no answer to why the earth, or any given planet, is a particular distance from its host star. It can be at any distance and it's just the vagaries of the way in which a planetary system forms that determines whether a planet is at one place or another. The real question that Kepler should have been asking is, why do we humans find ourselves on a planet 93 million miles from our star instead of any of the other possible distances that could be realized? That's a question we can answer. We live at that distance, on that planet, where the conditions are hospitable to our form of life. That is the right question and that is the right answer.

RAZ: Which brings us to Brian Greene's big idea that, like Kepler four centuries earlier, we may be asking the wrong question about the nature of our expanding universe and the dark energy driving that expansion, the number, the amount. It might not be unique or even special because our universe - it could just be one of many universes collected together in one massive multiverse. And there are mathematical models that lay this out.

GREENE: If you study the multiverse, mathematically, you find that it's very natural that the other universes would also have dark energy but that the amount of dark energy they have would be different from the amount of dark energy that we have. And, in fact, if you examine a realm that has a very different amount of dark energy, the repulsive push from a greater amount of dark energy is so strong that it blows apart planets and stars before they even get a chance to form. So the only universe that we humans can exist in is a universe where there's a small amount of dark energy, 122 zeroes and then a one, allowing galaxies and planets to form. So it's a new kind of explanation, one that makes people uncomfortable. It's sort of attributed, a little bit, to a cosmic accident. All of these possibilities are out there and we live in the one where the conditions are hospitable to our form of life.

(SOUNDBITE OF BRIAN GREENE TED TALK)

GREENE: So to pull it all together, we need a mechanism that can actually generate other universes because such a mechanism has been found by cosmologists trying to understand the Big Bang. You see, when we speak of the Big Bang we often have an image of a kind of cosmic explosion that created our universe and set space rushing outward. But there's a little secret. The Big Bang leaves out something pretty important: the bang. It tells us how the universe evolved after the bang but gives us no insight into what would have powered the bang itself. And this gap was finally filled by an enhanced version of the Big Bang Theory. It's called inflationary cosmology, which identified a particular kind of fuel that would naturally generate an outward rush of space. The fuel is based on something called a quantum field but the only detail that matters for us is that this fuel proves to be so efficient that it's virtually impossible to use it all up, which means, in the inflationary theory, the Big Bang giving rise to our universe is likely not a one-time event.

(SOUNDBITE OF SONG IN LATIN)

RAZ: But if we just happen to live in one universe, I mean, if our universe is infinite - if it's infinite - it's the totality of existence. So how could you have other universes beyond it?

GREENE: Well, remarkably, you can and the math is very clear on this, that you can have infinite universe, after infinite universe, after infinite universe all populating a larger cosmic domain that would embrace them all. It could be that there's a big, giant, cosmic bubble bath, with bubble, after bubble, after bubble, being universe, upon universe, upon universe. Now the strange idea there, I'll just interject, is when I say a bubble for a universe, that seems to suggest that it's finite in size. But the wonders of general relativity are such that a realm can appear finite from the outside but if you're in that universe, in that bubble, it can appear infinitely big. And that's how you can have a collection of infinitely big universes all within some larger domain. So the bottom line is, our best theories of how our universe got started suggest that the Big Bang may not have been a unique event, that there may be many Big Bangs, each giving rise to its own expanding domain, with our universe just being one of many.

RAZ: Physicist Brian Greene.

Here's one more thing to think about. The next time you're looking up at the sky, the next time you soak in the wonder, the possibility out there, the math that predicts the multiverse also suggests it could contain an infinite number of universes, which means, theoretically, there could be copies of our universe somewhere out there, exact copies of identical universes that were born under identical conditions and unfolded and evolved in exactly the same way ours did. Copies of the Earth and the solar system and even individual people and individual events, like this conversation with Brian Greene.

GREENE: In a vague sense, yes. The multiverse tells us, in ways that, look, we find mind-blowing ourselves and we're not in any way sure it's right; but the multiverse idea suggests that that notion, that there might be other worlds out there, other universes out there, maybe with copies of ourselves - that potentially could be true.